In a disk-based data storage device, such as a hard disk drive, data is typically recorded to and read from a plurality of data tracks. Generally, the “nominal” (i.e., desired or ideal) track shape is circular, and “nominal” tracks are concentric about the disk axis of rotation.
Typically, as part of a manufacturing or setup procedure (prior to normal use for data read/write), a hard disk drive is provided with a plurality of servo “bursts.” The purpose of these bursts is to provide location information to components of the head-positioning system, typically a servo-type control system, which can allow the servo system to maintain the read/write head at a desired radial position as the disk rotates, i.e., to “follow” a desired data track. Although the present invention can be used in connection with any of a number of servo burst systems, those with skill in the art will understand how to use the present invention using any of a number of servo burst systems based on a description of a particular servo burst configuration. In this configuration, a plurality of servo bursts is positioned around the track. Typically, the bursts are circumferentially aligned, from one track to the next, defining a plurality of radial “spokes.”
In one particular servo burst configuration, for each track, or group of tracks, each spoke contains four bursts (plus other information such as track number, etc). For purposes of the present disclosure, these four bursts will be referred to as A, B, C and D burst components. Each component has a nominal radial width of one track width, with the components being configured such that two of the components (e.g., the “A” and “B”) components have their “radial” midlines aligned with the nominal track centers and the other components (e.g., “C” and “D”) have their nominal edges aligned with the track centers. As the servo bursts pass under the read head, each component will result in a detected signal with a magnitude between minimum and maximum values and, accordingly, four signal amplitudes A, B, C, and D will be obtained. If the servo burst components are positioned substantially so as to define concentric and equidistant tracks, and have substantially their nominal sizes and shapes, the relative magnitudes of the servo burst signals A, B, C, and D will be indicative of whether the read head is at the nominal track center and, if not, the four magnitudes can be used to compute a position error signal (PES). In practice, the position of the burst components has repeatable and nonrepeatable components. Non-repeatability of the burst positioning results in irregular burst shapes and sizes. If all bursts are the same size and shape but their positions are such that all tracks are concentric and equidistant although non-circular then there will be repeatable run out but no instability. The position error signal, typically with, some correction or manipulation, including as described below, can be used as an input to the tracking servo system in a manner so as to drive the read head toward the desired radial position and, thus, achieve the desired tracking.
When the servo bursts are written on the disk, typically using a servo writing apparatus and procedure, it is possible that the servo burst components may sometimes have a size, shape or position which departs from the desired concentric, equidistant configuration. If the deviations are relatively small (e.g., typically no more than about +/−0.07 of the track width), it is possible, in general, to use the PES signal to provide a feed forward control input to the servo system that can reduce or eliminate (typically higher-frequency) recurring tracking deviations (repeatable run out) to achieve a certain degree of improvement in track positioning. Theoretically, all frequency harmonics of the repeatable run out can be eliminated but various environmental changes like temperature and disk slip make the first few harmonics change so that one-time correction for these is not effective. However, for larger-magnitude deviations of burst component shape, size or position, non-linearalities are introduced during track position detection (TPD) which become significant enough that instability is created in the servo loop which may render the track unusable for reading and writing. When a track of this type is detected, it is typically mapped out from the drive format (tagged as unusable). If there are more than a threshold number of tracks which are mapped out (typically a few tenths of a percent), the entire drive may be considered unusable.
Although previous methods did not adequately deal with seriously-distorted servo burst components, it is believed that it was often a sufficiently infrequent occurrence and that it was not considered a substantial problem in the industry. The present invention, however, addresses this issue of burst component distortion at least partially because of a recognition that as data density increases, and track width decreases, the percentage of track width that a given absolute amount of burst component distortion represents will also increase. Accordingly, it would be useful to provide a method and apparatus which can effectively deal with burst component distortions that might otherwise cause servo instability, or, in some other way, contribute to undesired mapping out of data tracks or unusability of disk drives. It would be particularly useful to provide a method and apparatus for dealing with distorted servo burst components which can be implemented in a relatively straightforward fashion, preferably being capable of implementation substantially by changes in software.